Stabilized thermoplastic cellulose ether composition



Patented Dec. 26, 1950 STABILIZED THERMOPLASTIC CELLULOSE ETHER COMPOSITION Walter E. Gloor, Wilmington, Del., and George H.

Pyle, Hopewell, Va., assignors to Hercules Powder Company, Wilmington, Del., a corporation of Delaware No Drawing. Application September 25, 1948, Serial No. 51,321

Claims. 1

This invention relates to the stabilization of cellulose ethers and, more particularly, to the production of cellulose ethers and their compositions which retain their viscosity and good color during and after exposure to heat, light, oxidation, weathering and other degrading influences.

Cellulose ether compositions and particularly ethyl cellulose compositions have found considerable usefulness where the properties of toughness, dimensional stability and flexibility at extremely low temperatures are desirable However, when subjected to elevated temperatures, prolonged exposure to ultraviolet light or aging in general, there has been a discoloration and/or viscosity degradation which has limited their usefulness to a considerable extent. Thus, compositions depositing a colorless mass from solution have given relatively dark amber masses much like tortoise shell upon being molded at temperatures of the order of 200-300 C. In addition, there has been considerable loss of viscosity, i. e., degradation, leading to brittle molded products. Similar discoloration and loss of viscosity, strength, and flexibility also result from prolonged storage or exposure to ultraviolet light and weathering. The problem of stabilization is particularly difficult where relatively high heats are required, as in fast molding, or when the material is exposed for prolonged periods to severe weathering conditions.

In the past, viscosity stabilization has been effected to some extent by the addition of certain amines and certain phenolic bodies. In most cases these have tended to add color themselves or have contributed to the formation of color in the cellulose ether. In the few cases where color development has been less than that obtained where no stabilizers were used, the extent of color improvement has been insufficient and has been obtained largely under moderate, rather than high, heat conditions.

Some attempts have been made to improve color by treatment with bleaching agents and by treatment with acids. Where such improvements in color have been retained in the molding operation, they have resulted in such extreme loss of viscosity that a relatively useless molded product results. Even mildly bleaching, slightly acidic agents, such as sulfur dioxide, have been found to cause crazing, scorched dark spots, brittleness; i. e., local or general degradation at molding temperatures of 200 C., and above, either immediately or on normal aging. As a result, it has not been possible to prepare clear, colorless, molded ethyl cellulose articles or even to prepare molded ethyl cellulose articles in pale or pastel shades. Ethyl cellulose molded articles have, therefore, been made only in pigmented Or dark colors.

Now, in accordance with this invention, it has been found that ethyl cellulose and other thermoplastic cellulose ethers and their compositions may be effectively stabilized against discoloration and viscosity degradation, when exposed to heat and weathering, by adding to or incorporating with the cellulose ether a small amount of p-tert-amylphenol, alone or in combination with a sulfur dioxide-producing compound. When used alone, the p-tert-amylphenol exerts a very definite stabilizing effect on cellulose ethers and their compositions, protecting them from discoloration and degradation when exposed to heat, light, air, weathering and other degrading influences. Optimum results are obtained, however, when the p-tert-amylphenol is used in combination with small amounts of a sulfur dioxide-producing compound. Thus, in accordance with this invention, cellulose ethers and their compositions may be molded at elevated temperatures such as 200 C. and above to form tough, shaped articles very greatly improved in color after molding as compared with previous molded compositions of this nature. As a result it is now possible to successfully prepare clear, relatively colorless moldings and moldings in light and pastel shades.

In view of the acidic nature of sulfur dioxide and in view of the fact that phenolic bodies are, in general, of an acidic rather than basic nature, it is surprising and unexpected that the use of p-tert-amylphenol alone or in combination with sulfur dioxide in cellulose ether compositions at molding temperatures brings about formation of tough, undegraded products of improved color. Heretofore, it has been the experience in this field that acids caused severe degradation, sometimes accompanied by discoloration, when cellulose ethers were exposed to heat or aged. Furthermore, although considerable improvement in stability characteristics and particularly color are obtained by the use of p-tert-amylphenol alone, it has been found that optimum stability improvement is obtained when p-tert-amylphenol and a sulfur dioxide-releasing compound are used in combination,

The invention will be illustrated by the examples of preparation of molding compositions and preparation of molded articles therefrom which follow. All parts and percentages given are by weight.

EXAMPLE 1 Ethyl cellulose having 46.8% to 48.5% ethoxyl content and 100 cps. viscosity was dissolved in 80:20 toluene:a cohol. p-tert-Amylphenol was added to a portion of the solution to give a composition containing 1% p-tert-amylphenol, based on the weight of the ethyl cellulose. Films about 3 mils in thickness were then cast from the composition, and portions of the films thus formed were heated for 48 hours at 120 C. in air. Films of the ethyl cellulose containing no stabilizer were also heated under the same conditions. Viscosities of the samples before and after heat treatment were determined and from these figures the viscosity retention was obtained. Flexibi'ity and discoloration were also noted.

Table I below compares the results obtained with the stabilized and unstabilized ethyl cellulose.

7 TABLE I Evaluation of p-tert-amylphenol as stabilizer for ethyl cellulose after heating for 48 hours at 120 C. in air.

Per Cent Viscosity Retention Sample Flexibility Discoioration Unstabilized p-tert-Amylphenol Flexible... do Nl TABLE II Evaluation of p-tert-amylphenol as stabilizer for ethyl cellulose after exposure in the Fade- Ometer for 50 hours Per Cent Discoloration Viscosity Retention Sample Flexibility Unstabilizcd p-tert-Amylphenol (1%)- Brittle Flexible.

These data illustrate that a small amount of p-tert-amylphenol in an ethyl cellulose film which is exposed 50 hours in the Fade-Ometer effectively stabilizes the ethyl cellulose with respect to both film flexibility and viscosity reten tion.

EXAMPLE 2 The following ingredients were colloided on a two roll-mill at 290-310 F. for minutes, dried, chipped into a molding powder, and molded into discs at 400 F. for 10 minutes.

Parts Ethyl cellulose (44.5 to 45.5% ethoxyl, 100

cps. viscosity) 85 Dibutyl phthalate 15 Similar compositions were prepared and molded containing (1) 2% p-tert-amylphenol, (2) 0.3% butadiene sulfone, and (3) 2% p-tert-amylphenol and 0.3% butadiene sulfone.

All samples were exposed to the 8-1 sun lampfog box cycle (2 hours in the fog chamber, 2 hours irradiation under the 6-1 lamp, 2 hours in the fog chamber, 18 hours irradiation) for hours. Color of discs after moding and color and condition of discs after exposure to 8-1 sun lamp-fog box cycle were determned and are set forth in Tables III and IV below. Colors were measured on the spectrophotometer, and values are expressed as luminous transmittance and as the red trichromatic coefiicient. The former value represents the amount of light that each disc will transmit, and the red coefficient represents the per cent of red in the total color value of the sample.

TABLE III Color of discs after molding (spectrophotometer) TABLE IV Color and condition of discs after exposure to 8-1 sun lamp-fog box cycle (spectrophotometer color values) Lumi- Trichro nous matic Sample Trans- (oeili- Condition mittance. ciont Per Cent (Red) Unstabilized 1. 9 0017 Surface badly crystallized and disintegrating.

2% p-tert-Amylphenol 16.5 .4034 Fine surface craze.

0.3% Butadiene sulfone. 14. 8 .4962 Surface badly crystallized and disintegrating.

2% p-tert-Amylphcnol 18.8 .4452 Fine surface craze.

and 0.3% Butadiene sulfone.

A higher percentage value for luminous transmittance in the tabulated data signifies better clarity with more total light being transmitted through the sample than a lower value signifies. A lower value for the red trichromatic coefficient in the tabulated data signifies less red in the total color value of the sample and therefore a more nearly colorless sample. Thus, these data illustrate that ethyl cellulose plastic discs containing a small amount of p-tert-amyiphenol alone or in combination with butadiene sulfone transmit a great deal more light and show much better resistance to discoloration than an unstabilized ethyl cellulose plastic disc, both immediately after molding and after exposure for 120 hours to the 8-1 sun lamp-fog box cycle. Furthermore, the surface condition of ethyl cellulose plastic discs containing a small amount of p-tert-amylphenol alone or in combination with butadiene sulfone was in much better condition than either the unstabilized ethyl cellulose disc or the ethyl cellulose disc containing butadiene sulfone alone. The surface of the ethyl cellulose discs which did not contain any p-tert-amylphenol was badly crystallized and disintegrating in contrast to a fine crazing without surface disintegra- EXAMPLE 3 To 175 grams of flake ethyl cellulose containing 0.35% by weight of butadiene sulfone was added 62 grams of dibutyl phthalate and 7 grams of p-tert-amylphenol, and the mixture was placed on a two-roll mill with the rolls at 310 110 F. With the mill running, an additional 1'75 grams of the same flake ethyl cellulose was added and the material was milled at 300 to 320 F. for minutes. The composition was then stripped from the rolls as a well-colloided plastic sheet, broken into pieces, and ground to about 8 mesh with an Abb cutter. Exactly 200 grams of this molding powder was placed in a glass-top quart jar with the top in place but not sealed. The jar and its contents were heated for 16 hours at 100 C.

in an oven. From this molding powder a plastic disc 2 inches in diameter and 0.065 inch thick was compression molded in a conventional ring and plug mold, employing 5 grams of molding powder and molding at 2000 lb. p. s. i. at 200 C. for 5 minutes.

A similar molding powder and molded disc were prepared by the same procedure from the same flake ethyl cellulose but without any p-tertamylphenol.

Viscosity measurements were then made on the original flake ethyl cellulose, on the molded disc of ethyl cellulose plastic composition containing p-tert-amy phenol and on the molded disc of ethyl cellulose plastic composition without the p-tert-amylphenol. Viscosity values for the molded plastic compositions were corrected for plasticizer content in order to place all viscosities on a comparabe basis with the viscosity of the original flake ethyl cellulose as the reference standard.

Viscosities were measured at C. :01 C. on 5% by weight solutions of the original flake ethyl cellulose and of the molded plastic compositions in a solvent composed of 80 parts to uene and 20 parts denatured ethyl alcohol by weight. Viscosity values are given in Table V below.

TABLE V Evaluation of p-tert-amylphenol as stabilizer for ethyl cellulose after high temperature milling and molding operations Vi co ity Sample Cen tipoiscs) Original Flake Ethyl Cellulose 1 Containing 0.35% Butadicne dulfone by WQlEl'lt l. 71 Ethyl Cellulose 1 Containing 0 Butadiene Sulfone by weight in Molded Di c 12.4 Ethyl Cellulose 1 Containing 0.35% Butadiene Suli'one by weight plus 2% by weight of p-tert-Amylphenol in Molded Disc 64. 4

1 46.4% ethoxyl content.

and the like, are of the required thermoplastic type. Ethyl cellulose having an ethoxyl content between about 37% and about 52%, preferably between about 43% and about 48% and having a viscosity of at least about 20 cps, is particularly suitable. However, thermoplastic propyl cellulose, ethyl propyl cellulose, ethyl butyl cellulose, methyl ethyl cellulose, and benzyl cellulose are likewise useful in molding compositions, and, like ethyl cellulose, their molding compositions are very greatly improved in color of the molded product by proceeding in accordance with this invention. It is desirable that the cellulose ethers be so prepared or purified as to eliminate any substantial proportion of free acid. Thus, treatment with strong acids should be avoided unless there is subsequent neutralization of any free acid groups by basic ions, such as sodium, calcium, magnesium, cesium, copper, and the like.

The stabilizers according to the present invention may be incorporated before, during or after preparation of the cellulose ether compositions or while the cellulose ether compositions are being molded or otherwise formed into finished plastic articles. For example, p-tert-amylphenol alone is used, the cellulose ether may be dissolved in a suitable solvent to which the p-tert-amylphenol is added, after which solvent is removed. Aternatively, the cellulose ether may be suspended in water or a swelling medium, such as aque ous alcohol and a solution of the stabilizer added to the suspension for absorption by the cellulose ether. Again the stabilizer may be added during a tumbling, stirring, or other similar operation for distributing the stabilizer through the granular cellulose ether. Incorporation of the stabilizer at some stage of manufacture is desirable, since it frequently happens that the cellulose ether is stored for considerable periods before use, and the presence of an eiiective stabilizer prevents any substantial degradation due to aging.

The stabilizer may also be incorporated during preparation of the product in which the cellulose ether is eventually employed. For example, in the preparation of cellulose ether lacquers, molding powders, etc., the stabilizer may be added to the finished lacquer or to the lacquer solvent during or prior to dissolving the other lacquer ingredients; to the mixture of ingredients prior to or during formation of a molding powder, etc.

The amount of p-tert-amylphencl utilized may vary from about 0.5% to about 5.0% of the weight of the cellulose ether. Preferably, it is employed in amounts between about 1% and about 3%, 1% usually being the optimum quantity to obtain satisfactory stability and other required characteristics. Larger amounts than about 5% produce no substantial additional stabilizing effeet, while amounts below about 0.5% usually do not create the desired degree of stabilization.

When sulfur dioxide is included substantially similar amounts of p-tert-amylphenol will be utilized in order to obtain maximum stability characteristics. Sulfur dioxide may be provided as such at the time of molding the cellulose ether composition containing the p-tert-amylphenol, by passing a small stream of sulfur dioxide into the molding chamber or into a chamber or hopper by which the thermoplastic composition is led to the molding chamber. A similar result is obtained by subjecting a molding powder of the cellulose ether composition to an atmosphere of sulfur dioxide, whereby it absorbs some of the sulfur dioxide, and then molding the composition by compression, injection, or extrusion under heat before the absorbed sulfur dioxide has been lost.

Usually it is more convenient to incorporate in the molding composition a compound which will decompose at the molding temperature to form sulfur dioxide, and p-tert-amylphenol, and then molding the composition under heat. Temperatures utilized in molding will be at least sufficient to cause release of sulfur dioxide from the particular compound used. Generally these will be of the order of about 200 C., although tem peratures above or below this figure may be used. By this method, a small amount of sulfur dioxideis released under the heat of the molding operation. Thus, there is provided a thermoplastic cellulose ether molding composition containing. both the compound capable of releasing sulfur dioxide and the p-tert-amylphenol. Such a composition is capable of being molded to strong, tough articles Which may be of pale color or a pastel shade or of very greatly reduced color in the case of clear articles.

The sulfur dioxide-releasing compound may be incorporated inv the cellulose ether at the time it is manufactured, or it may be incorporated during the preparation of the molding composition. Either of the latter procedures has the advantage of providing the cellulose ether with a color protective influence during the milling needed to form a molding powder.

Any compound which is substantially colorless,

which does not discolor under heat, and which does not in itself function as an acid stronger than sulfurous acid is suitable. The sulfones of aliphatic compounds, such as those derived frombutadiene, piperylene, isoprene, Z-methyl penta diene, amyldiene, or other diene hydrocarbons, are particularly useful since they release sulfur dioxide freely at 200 C. without leaving any appreciable residue. Polypropylene sulfone, polyalkyl-polysulfones, generally, and cyclic dipro pylene sulfone are effective. Aliphatic sulfonates provide another group of compounds which, in most cases, give off sulfur dioxide at 200 0.; such sulfonates as sodium sulfodiethyl succinate, sodium sulfodipropyl succinate, sodium sulfodi butyl succinate, sodium sulfodiamyl succinate, sodium sulfodihexyl succinate, sodium sulfodioctyl succinate, sodium sulfodilauryl succinate, equivalent potassium compounds, and similar de rivatives of substituted succinates, have been found particularly suitable since the residue is quite compatible in the cellulos ether compositions and, in fact functions as a plasticizer. These compounds may be prepared by condensation of sodium bisulfite with the corresponding esters of maleic acid. Any aliphatic or alicyclic sulfonate which will release sulfur dioxide upon heating is suitable. Allsyl and other organic sulfites, for example, dimethyl sulfite, diethyl sulfite, dibutyl sulfite, methyl bisulfite, ethyl bisulfite, acetone bisulfite, normal-heptaldehyde bisulfite, and sodium formaldehyde sulfoxylate, were found to function by release of sulfur dio:' ide at 200 C. Inorganic compounds, such as sodium bisulfite sodium meta-bisulfite, potassium bisulfite, calcium bisulfite, sodium hydrosulfite, and the like, which are capable of releasing sulfur dioxide upon heating at 200 C. have also been found suitable. The inorganic materials are of use in pale pigmented melding compositions but tend to add haze to compositions intended for clear moldings.

The quantity of sulfur dioxide or sulfur dioxidcreleasing material will, in general, be quite small but may vary considerably with the particular compound uti ized. It is most easily expressed. on the basis of the content of sulfur dioxide in free or combined form, and, on this basis, at least about 0.005% by weight of sulfur dioxide based upon the cellulose ether in the molding composition should be present during the molding opera tion in order to obtain improvement in color of the molded article. Preferably, a quantity between about 0.05% and about 0.5% of sulfur dioxide based on the weight of the cellulose ether willbe utilized, it being understood that this amount represents only the sulfur dioxide content of the actual compound present. In general, no more than about 0.2% of sulfur dioxide in loosely combined form need be present, and in view of the possibility of corrosion of equipment by sulfur dioxide, no more than this amount will ordinarily be used. In any case, no more than 2% of combined or free sulfur dioxide should be used because of development of objectionable odor and degradation, scorching, etc. in the finished plastic.

Where gaseous sulfur dioxide is used, only a small amount need be passed through the molding composition as absorption of only very small amounts is needed. Where butadiene sulfone is used, 0.2 to 0.5% on the weight of the cellulose ether (equivalent to 0.11% to 0.27% sulfur dioxide) is an optimum quantity, some of the sulfur dioxide being released in milling to form molding powder and the remainder, or some of the remainder being released in molding. However, 0.01% to 2.0% of the sulfone, based on the cellulose ether, may be used. When sodium sulfodioctyl succinate or homologous compound is used, 0.3% to 1.0% of the compound is optimum, although 0.05% to 4.0% may be used. (This represents 0.005% to 0.29% sulfur dioxide based on the cellulose ether.) The amount may be less than in the case of butadiene sulfone, as indicated by the figures, because there is less sulfur dioxide released in milling to form molding powder, and, therefore, relatively more sulfur dioxide avaiiable in the molding operation. It will be appreciated that, depending on particular compounds, compositions, and uses, the optimum quantities will vary considerably.

It will be understood. that plasticizers, such as dibutylphthalate, diethyl phthalate, butyl' stearate, triphenyl phosphate, tricresyl phosphate, raw castor oil, nonvolatile mineral oils, methyl phthayl ethyl glycolate, hydrogenated methyl abietate, and the like, may be incorporated with the cellulose ether as usual in the preparation of plastic masses. Likewise, resins, such as the oilsoluble phenol aldehyde condensates, ester gum, hydrogenated glycerol abietate, pentaerythntol abietate, rosin, and oil-modified alkyd resins, would also be included although, as a ruie, these substances are not used extensively in plastics intended for molding. Similarly, waxes, such as paraffin, microcrystalline petroleum waxes, carnauba lWflX, candelilla wax, montan wax, and Japan wax, may also be included. Pigments, dyes, and fillers may also be included.

The method in accordance with this invention includes shaping with the aid of heat by any mechanical modification. Thus, shaping may be by compression molding under heat, injection molding, or by extrusion, drawing, and the like. Tem-- p-eratures may vary from to 300 C. The invention is particularly valuable in permitting molding at the relatively high but efficient and frequently necessary temperatures of the order of -250 C.

It will be appreciated that the compositions in accordance with this invention are also useful where heat is not essential for shaping but where a composition may be subjected to relatively high temperatures or to moderately high temperatures for long periods of time. Thus, the compositions retain good color and stability over long periods of exposure to heat and light in the form of lacquer films, electrical insulation, impregnated and coated fabric, and in film or sheeting,

It has been found that the compositions in accordance with this inventon can be molded at quite high temperatures without the formation of rather dark color as normally encountered with cellulose others. The compositions are also stabilized against degradation in the form of severe viscosity drop. Stability of this nature preserves toughness. Furthermore, the compositions are capable of withstanding ultraviolet light of the type encountered in exposure to sunlight and sun lamps for long periods without development of dark spots, discoloration,- and other degradation. The invention permits the preparation of clear and pastel shades in thermally molded cellulose ether compositions.

Viscosities given throughout this specification are defined in terms of centipoises as determined on solutions of the cellulose ether in 80:20 toluenezalcohol at 25 C. The per cent viscosity retentions were calculated by determining the viscosities of a given sample before and after treatment, such as heat-treatment, and dividing the latter by the former.

The 8-1 sun lamp-fog box cycle is fully described in the Government publication dated January 24, 1944, and entitled Federal Specification for Plastics, Organic-General Specifications--Test Methods under test LP406c. The cycle includes exposure in a fog chamber for two hours, two hours irradiation under an 8-1 lamp,

two hours additional exposure in the fog chamber, followed by eighteen hours irradiation. The cycle may be repeated any number of times.

What we claim and desire to protect by Letters Patent is:

1. A stabilized cellulose ether composition comprising a thermoplastic cellulose ether and from about 0.5% to about 5% of p-tert-amylphenol, said composition being stabilized with respect to viscosity and color during and after exposure to heat, light, oxidation, and weathering by virtue of the presence therein of said p-tert-amylphenol.

2. A stabilized cellulose ether composition comprising a thermoplastic ethyl cellulose and from about 0.5% to about 5% of p-tert-amylphenol, said composition being stabilized with respect to viscosity and color during and after exposure to heat, light, oxidation, and weathering by virtue of the presence therein of said p-tert-amylphenol.

3. A stabilized cellulose ether composition comprising a thermoplastic cellulose ether, a sulfur dioxide-producing compound which is substantially colorless, which does not discolor under heat, which does not in itself function as an acid stronger than sulfurous acid and which releases sulfur dioxide at 200 C., in an amount between about 0.005% and about 0.5% of the weight of the cellulose ether on the basis of the content of combined sulfur dioxide, and from about 0.5% to about 5% of p-tert-amylphenol, said composition being stabilized with respect to viscosity and color during and after exposure to heat, light, oxidation, and weathering by virtue of the presence than sulfurous acid and which releases sulfurdioxide at 200 C., in an amount between about 0.005% and about 0.5% of the weight of the ethyl cellulose on the basis of the content of combined sulfur dioxide, and from about 0.5% to about 5% of p-tert-amylphenol, said composition being stabilized with respect to viscosity and color during and after exposure to heat, light, oxidation, and weathering by virtue of the presence therein of said p-tert-amylphenol and said sulfur dioxide-releasing compounds.

- 5. A stabilized cellulose ether composition comprising a thermoplastic ethyl cellulose, butadiene sulfone in an amount between about 0.01% and about 2.0% of the weight of the ethyl cellulose, and from about 0.5% to about 5% of p-tertamylphenol, said composition being stabilized with respectto viscosity and color during and after exposure to heat, light, oxidation, and weathering by virtue of the presence therein of said p-tert-amylphenol and said butadiene sulfone.

6. A stabilized cellulose ether composition comprising a thermoplastic ethyl cellulose, a sulfodialkyl succinate which is substantially colorless, which does not discolor under heat, which does not in itself function as an acid stronger than sulfurous acid and which releases sulfur dioxide at 200 C., in an amount between about 0.005% and about 0.29% of the weight of the ethyl cellulose on the basis of the content of combined sulfur dioxide, and from about 0.5% to about 5% of p-tert-amylphenol, said composition being stabilized with respect to viscosity and color during and after exposure to heat, light, oxidation, and Weathering by virtue of the presence therein of said p-tert-amylphenol and said sulfur dioxidereleasing compound.

'7. A stabilized cellulose ether composition comprising a thermoplastic ethyl cellulose, sodium sulfodioctyl succinate in an amount between about 0.05% and about 4.0% of the weight of the ethyl cellulose, and from about 0.5% to about 5% of p-tert-amylphenol, said composition being stabilized with respect to viscosity and color during and after exposure to heat, light, oxidation, and weathering by virtue of the presence therein of said p-tert-amylphenol and said sodium sulfodioctyl succinate.

8. A stabilized cellulose ether composition comprising a thermoplastic ethyl cellulose, an organic sulfite which is substantially colorless, which does not discolor under heat, which does not in itself function as an acid stronger than sulfurous acid and which releases sulfur dioxide at 200 C. in an amount between about 0.005% and about 0.5% of the weight of the ethyl cellulose on the basis of the content of combined sulfur dioxide, and from about 0.5% to about 5% of p-tert-amylphenol, said composition being stabilized with respect to viscosity and color during and after exposure to heat, light, oxidation, and weathering by virtue of the presence therein of said p-tert-amylphenol and said sulfur dioxide-releasing compound.

9. A stabilized cellulose ether composition c mprising a thermoplastic ethyl cellulose. diethyl sulfite in an amount between about 0.005% and on the basis of the content of combined sulfur 10. A composition comprising a thermoplastic cellulose ether, a sulfur dioxide-producing com-- pound which is substantially colorless, which does not discolor under heat, which does not. in itself function as an acid stronger than sulfurous acid and which releases sulfur dioxide at 200. C;, in. an amount at least about 0.005% and not more than 2% of the weight of the cellulose ether on the basis of the content of combined sulfur dioxide, and from about 0.5% to about 5% of p-tert-amylphenol, said composition being stabilized with respect to. viscosity and color during and after exposure to heat, light, oxidation, and weathering by virtue of the presence therein of said p-tert-amylphenol and said sulfur dioxiderel'easing compound. 7,

WALTER E.. GLOOR.

GEORGE H; PYLE.

1 2 REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Num er Name Date 2,109,593 Macht Mar. 1, 1938 2,275,716 Bachman Mar. 10, 1942 2,354,745 Dreyfus Aug. 1, 1944 2,407,209 Swan Sept. 3, 1946 2,465,915 Myles Mar. 29, 1949 FOREIGN PATENTS Number Country Date 741,975 France Feb. 24, 1933 361,341 Great Britain Nov. 16, 1931 Bacher et al.: Rec. des 'I'rav. Chim. des Pay- Bas (1935), pp. 538-to544.

Certificate of Correction Patent No. 2,535,357 December 26,- 1950 WALTER E. GLOOR ET AL.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 6, line 25, after the Word and comma example, insert when; column 7 line 65, after sodium bisulfite insert a comma; column 11, line 9, before composition insert stabilized cellulose ether;

and that the said Letters Patent should be read as corrected above, so that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 6th day of March, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commissioner of Patents. 

1. A STABILIZED CELLULOSE ETHER COMPOSITION COMPRISING A THERMOPLASTIC CELLULOSE ETHER AND FROM ABOUT 0.5% TO ABOUT 5% OF P-TERT-AMYLPHENOL, SAID COMPOSITION BEING STABILIZED WITH RESPECT TO VISCOSITY AND COLOR DURING AND AFTER EXPOSURE TO HEAT, LIGHT, OXIDATION, AND WEATHERING BY VIRTUE OF THE PRESENCE THEREIN OF SAID P-TERT-AMYLPHENOL. 